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Old 22nd Nov 2021, 2:34 pm   #1
HBWOODY
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Default VCM 163 setting up procedure.

The VCM163 Service Manual setting up procedures produces the correct results until I reach (v)/(w). The expected readings are I.5m/A, 5mA, 15mA, 50mA. The readings I get are 1.59m/A, 5.3m/A, 16.3m/A, 64m/A.
I used resistors in parallel to get the correct measurements for the positions 3/10/30/100 on the m/A switch. Not sure what to do to solve this problem.
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Old 23rd Nov 2021, 2:13 pm   #2
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Default Re: VCM 163 setting up procedure.

Did you follow the steps correctly and short the 3.9k resistor for the 30mA & 100mA ranges, changed the Anode voltage as stated and adjusted the 1K potentiometer to set the meter to full scale deflection for each of the ranges?

What external meter did you use?

If you did so and the Anode voltage stayed stable your shunt resistors are probably incorrect.
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Old 1st Dec 2021, 8:20 pm   #3
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Default Re: VCM 163 setting up procedure.

I followed the steps correctly. I used my Fluke 87V for measuring the m/A. The shunt resistors were used to obtain the correct values for R11-R14. Would the shunt resistors interfere with the m/A meter readings?
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Old 1st Dec 2021, 8:37 pm   #4
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Default Re: VCM 163 setting up procedure.

Have you checked the Ia-meter to see if it has the correct FSD current of 50uA and internal resistance of 2.4K +/-1%?

You can also replace the Ia-meter with a 2.4K resistor in series with your Fluke 87V to set the current to 50uA with the 1K potentiometer for each test setting, then put your meter in place of the 2.4K resistor and see how far it swings. You might have to lower the 2.4K resistor somewhat to compensate for the internal resistance of the Fluke 87V on the uA range (I don't know the internal resistance on that range).

Have you checked D1, D2, R1 & R2 and also looked at the waveform with a scope during each test to see if the half wave rectified voltage changes during loading? Those original Lucas rectifier diodes are known to behave strangely when they go bad.

Extra shunt or series resistors to R11-R14 shouldn't affect anything unless they are faulty in some way.
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Old 6th Dec 2021, 8:33 pm   #5
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Default Re: VCM 163 setting up procedure.

HBWOODY said:
Quote:
I used my Fluke 87V for measuring the m/A.
Iv'e been trapped by this some time ago when working on a MK3, maybe the 163 is the same - you should use the specified meter as the waveforms are just half-wave rectified AC - no smoothing cap so the DC dynamics of the Fluke are all wrong - I think they spec's an AVO8 or similar.
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Old 6th Dec 2021, 9:13 pm   #6
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Default Re: VCM 163 setting up procedure.

The Fluke 87V is perfectly all right to use for these measurements as it is a proper True RMS reading meter that measures half wave rectified sinusoidal voltages absolutely correctly in this circumstance.

However older "True RMS" meters like the Beckman RMS3030 does not measure half wave rectified sinusoidal voltages correctly. Some older Fluke meters also show incorrect measurements so you really need to use a proper True RMS meter for these readings.

You can check the BULL-DMMglossary-E.pdf in the thread below on how RMS is related to Mean DC voltages and also check the Fluke 87V instruction manual below on how it measures half wave rectified sinusoidal voltages. There are limits to the crest factor but that is well within what the 87V can handle in this circumstance.

https://www.vintage-radio.net/forum/...7&postcount=23

https://dam-assets.fluke.com/s3fs-pu...tSyLsuYWEUp3SY
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Old 6th Dec 2021, 9:19 pm   #7
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Default Re: VCM 163 setting up procedure.

They spec'd an electronic AVOmeter model EA113 for the later VCMs, and as a consequence, the prices on the open market for the things is crazy and they can be hard to find. Gorgeous little analogue meters with amazingly wide ranges. BUT a bit prone to mis-reading if there is any RF around. Worth acquiring even if you don't have a VCM, but at those asking prices?

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Old 6th Dec 2021, 10:19 pm   #8
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Default Re: VCM 163 setting up procedure.

Hi Martin, apart from the true rms aspect of the meter measurement, would there be any effect from the possibly higher impedance of the fluke when measuring current?

Ed
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Old 7th Dec 2021, 6:54 am   #9
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Default Re: VCM 163 setting up procedure.

Hi, "Woody"

I am writing an article on the AVO Valve Tester, the VCM MKI~IV, CT160 and VCM163. It is currently some 10 pages, with half a page of references.

It will appear in Silicon Chip, Australia's premier electronics/hobby magazine.

You can get an idea of the depth and quality of my work here:

https://www.siliconchip.com.au/Issue...e+–+Quadruplex

I have my own photos for the Valve Tester, the VCM MKII and MKIV and the CT160, but not for the MKI, MKIII or the VCM163

Can you help with photos? I would like a resolution of 1200 pixels minimum, and good lighting without excessive glare or reflections.

I will be paid for the article. The publisher is able to pay a usage fee to you, and we always give full credit to any contributor.

Please let me know if you can help with this important piece of historical research and publishing.

Ian Batty, M.Ed.

Victoria, Australia
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Old 7th Dec 2021, 10:46 am   #10
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Default Re: VCM 163 setting up procedure.

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Originally Posted by Ed_Dinning View Post
Hi Martin, apart from the true rms aspect of the meter measurement, would there be any effect from the possibly higher impedance of the fluke when measuring current?

Ed
That depends on the current range used and the total burden voltage that range affects the circuit with but in general this is not a problem with modern good quality DMMs compared to older moving coil instruments.

There is always a burden voltage from any current measurement due to the internal resistance of the instrument irrespective of what type of instrument you use, an old AVO 8 or a modern Fluke 87V. Usually there is no need to adjust for this burden voltage, but if it calculates to a big percentage of the total voltage in your circuit then you might have to adjust for it.

Instead of measuring the current you could measure the voltage drop across a resistor in the circuit (possibly taking into account the normal 10M input resistance of the DMM if needed) and then just divide that voltage with the resistance to get the current flowing through that resistor.

In the case of measuring the current through the meter circuit in an AVO Mk IV you could replace the meter with a 3250 Ohm resistor and measure the voltage drop across that resistor (with a nominal 10M input resistance of a DMM the 3250 Ohm resistor would only need to be 3251 Ohm to compensate for that input resistance so it is negligible). So for a measurement of the red SET AC line at 35.775uA we should get a voltage of some 87.75mV (taking into account that there is an external/internal 10K shunt in the circuit so the total resistance across the 3250 Ohm resistor in the circuit is only some 2452.83 Ohm).
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Old 8th Dec 2021, 8:18 pm   #11
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Default Re: VCM 163 setting up procedure.

The meter is calibrated correctly and R1 and R2 are within specification. D1 and D2 measure as they should, would it be a good idea to replace them with new ones? I have rechecked the measurements lots of times and I am happy with 3/10/30 settings which fall within spec it is now only the 100 setting which needs 60m/A to reach F.S.D.
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Old 8th Dec 2021, 8:37 pm   #12
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Default Re: VCM 163 setting up procedure.

Ian
My VCM163's are stripped down for servicing at the moment when I am confident that all is as it should be I will reassemble them and take some photos for you.
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Old 8th Dec 2021, 9:28 pm   #13
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Default Re: VCM 163 setting up procedure.

Quote:
Originally Posted by HBWOODY View Post
The meter is calibrated correctly and R1 and R2 are within specification. D1 and D2 measure as they should, would it be a good idea to replace them with new ones? I have rechecked the measurements lots of times and I am happy with 3/10/30 settings which fall within spec it is now only the 100 setting which needs 60m/A to reach F.S.D.
If you aren't interested in a historically correct restoration you can replace the diodes.

I would test with new diodes by unsoldering one side of the original diodes and hooking up new ones with crocodile clips to check if anything changes.

Likewise would I use a 2.4K resistor in place of the instrument and measure the voltage across it (50uA for FSD results in 120mV across 2.4K) instead of measuring the current in the circuit. Adjust the potentiometer for 120mV across the 2.4K resistor, check how many mA that flows through the potentiometer and see if that corresponds to what you expect, this way you will see if the shunts are correct with all the wiring tested at the same time. You can also rotate switches and press buttons to see if they affect the readings without the risk of blowing the meter this way.
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Old 8th Dec 2021, 10:28 pm   #14
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Default Re: VCM 163 setting up procedure.

I forgot to mention that you should also check the two back to back diodes across the meter, I have come across one 163 where one of those diodes was bad, similar to the problem you had with your amplifier board.
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Old 9th Dec 2021, 2:05 am   #15
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Default Re: VCM 163 setting up procedure.

The meter in the VCM itself is not true RMS, it's just a rectifier meter. When it's reading Ia, it gets halfwave rectified AC, not quite half cycles of sinewave, they are distorted by the valve's transfer function.

So the waveforms are a bit odd, and the scaling on the VCM's meter is arranged to compensate for these odd waveforms. So neither a true RMS meter nor a rectifier meter with a scale for AC allowing for its rectifier, and assuming a sinewave and scaled to show RMS value under those assumptions is right. Fiddle factors are involved.

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Old 9th Dec 2021, 1:08 pm   #16
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Default Re: VCM 163 setting up procedure.

Martin
I did as you suggested with the 2.4k resistor. The testing was easier not having to worry about the meter and the results have given me more confidence in the meters performance as the results were identical so the problem looks to be in the area of R11.
I will replace R11 "which is actually 3 resistors" and let you know if that solves the problem.
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Old 9th Dec 2021, 3:07 pm   #17
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Default Re: VCM 163 setting up procedure.

Replaced R11 and now get a reading of 47mA which is much closer to specification. Will look out for some 1% tolerance resistors.
Different topic but to do with setting up a VCM163. When moving the needle to Cal position
with RV2 and RV1 I can get it no closer than 12 on the 0-20 scale of the mA/V meter. The meter is within specification.
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Old 9th Dec 2021, 3:22 pm   #18
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Default Re: VCM 163 setting up procedure.

Quote:
Originally Posted by HBWOODY View Post
Replaced R11 and now get a reading of 47mA which is much closer to specification. Will look out for some 1% tolerance resistors.
Different topic but to do with setting up a VCM163. When moving the needle to Cal position
with RV2 and RV1 I can get it no closer than 12 on the 0-20 scale of the mA/V meter. The meter is within specification.
Nice that you got a better result, don't forget to use somewhat temperature stable resistors as they do heat up a little while the measurement is performed and with poor resistors you will get a small resistance change that affects the measurements a tiny bit.

What voltage do you have across the 10 Ohm, red/brown posts on the circuit board, check resistor on the oscillator board and what voltage do you have across the output transformer?

What voltage do you get across the input connections of the amplifier when the switch is set to the Cal. position - it will vary of course but what do you get as a maximum voltage?
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Old 9th Dec 2021, 8:10 pm   #19
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Default Re: VCM 163 setting up procedure.

Measurements taken:-
Across 1 and 2 amplifier input, 1.3mV in Cal setting, 3.1mV in 0-6 setting.
Across red/brown posts, 15mV.
Across 4/5 oscillator board, 56mV
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Old 9th Dec 2021, 8:47 pm   #20
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Default Re: VCM 163 setting up procedure.

Did you make these measurements on the AC range of your Fluke87V?
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